Abstract
The effects of crystalline conversion from the anhydrous form of prednisolone (A-PD) to its hydrated form (C-PD) on prednisolone (PD) release from o/w type ointment was studied in vitro at 37° and 5°, by using a diffusion cell and X-ray diffractometer. The conversion of A-PD in the ointment (A-PD ointment) into C-PD started on incubation for 4 days at 37° and the A-PD was entirely converted into C-PD after about 18 days. The longer the time of storage of the A-PD ointment at 37°, the less the release of PD from the ointment. On the other hand, A-PD was entirely converted into C-PD in the ointment in only one day at 5° ; the amount of PD released from the ointment was not dependent upon the time of storage and showed a constant release pattern. The crystalline conversion from A-PD to C-PD in the ointment is apparently due to the deterioration of the o/w type emulsion (in the ointment) with the passage of time. When the emulsion deteriorates, the A-PD dissolves in water released from the emulsion, and accordingly hydrated prednisolone (C-PD) is deposited in the ointment to an increasing extent. The reduction of PD release from the ointment stored at 37° is apparently due to the different solubilities of A-PD and C-PD in water. It is suggested that emulsion type ointment should be stored under suitable conditions.
